FIT rate of power semiconductors - What are the main factors?

FIT rate of power semiconductors: What are the main factors?

FIT rate (FIT = Failure In Time) is a common indicator to express the reliability of a semiconductor. And, as we know, reliability and availability is crucial for almost any customer. Therefore, the legitime question is: what are the main factors influencing the FIT rate?

According to the definition, FIT is defined as number of failures in 10^9 operation hours. One can convert FIT into MTBF (Mean Time Between Failures) and vice versa.

There are several important factors that significantly impact the FIT rate. One of them is for example the installation altitude as described in our previous posts (e.g. [1]).

The factors affecting FIT rate are coming partly from the design and partly from the way how the VFD is operated. For example, ABB Semiconductors (now part of Hitachi Energy) developed a mathematical model that is valid for all their power semiconductors. The parameters include:

Blocking voltage
Junction temperature
Installation altitude

Each of these factors can increase or decrease the FIT rate. Therefore, when one factor pushes the FIT rate high, the other factor shall be adjusted to compensate or neutralize the effect and keep the targeted reliability figure. A typical example is a drive application at high altitude (e.g. industrial drives for mills or conveyors at mining sites in South America, traction converters in trains operating at high altitudes such as Tibet etc).

The design optimization of a power semiconductor is also a certain tradeoff between ruggedness and device losses. Therefore, a compromise satisfying the operational requirements needs to be found.

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